Make a cream sauce of 2 tablespoons butter, flour and condensed milk; blend in mustard and 2 tablespoons cheese. Simmer a moment and salt to taste. Mix a little of the sauce with egg yolk; stir quickly into sauce. Add 1 tablespoon butter and combine with crab meat, shrimp and mushrooms. Add sherry. Pour into 1 1/2-quart casserole. Sprinkle with cheese and dot with remaining butter. Bake in 400-degree oven 15 minutes or until well heated and brown on top. Serves 6.